The proposal is focused primarily on the effects of nicotine in the suprachiasmatic nucleus (SCN), locus of the circadian clock in mammals, and an important influence on arousal state control. Preliminary studies have shown that nicotine produces robust phase shifts of the circadian clock in vitro. This effect on the clock will be explored using a rodent model to determine the mechanisms of nicotine induced phase shifts and whether nicotine can entrain circadian rhythms both in vivo and in vitro. The clock resetting properties of nicotine need to be taken into account to fully understand the addictive characteristics of cigarette smoking. The pharmacological nature of the receptors at which nicotine exerts its effects in the SCN will be determined, as well as the intracellular mechanisms mediating the response. Particular attention will be paid to the alpha7 nicotinic acetylcholine receptor (nAChR) subunit, which is the only alpha subunit found within the SCN. This newly cloned subunit produces a novel nAChR with unique pharmacology, including high affinity for alpha-bungarotoxin (alpha-BTX) when expressed in oocytes. This work will be done primarily in the in vitro SCN slice preparation, which maintains a circadian rhythm of neuronal firing. This rhythm is strongly altered by the application of nicotine, providing an ideal System in which to examine the details of nicotine action. The effects of nicotine on the biological clock in whole animals will be examined in an automated system known as SCORE, which can measure locomotor activity, drinking, feeding, body temperature, EEG and other variables simultaneously from many animals. The ability of nicotine to entrain circadian rhythms in rats could have relevance to human addiction. Part of the addictive liability of nicotine may result from a kind of circadian self-medication in smokers, especially to augment arousal in the morning. Smoking may actually synchronize the rest-activity cycle in a fashion similar and perhaps complementary to that of light. Nicotine withdrawal, on the other hand, might result in the loss of such synchronization and concomitant difficulty in arousing at preferred clock times, surely an aversive consequence in a clock-conscious society. Furthermore, the smoker in withdrawal might be most prone to recidivism early in the morning. The final specific aim of this proposal examines the effect of nicotine on the fetal SCN. A recent report has shown that cocaine administration in pregnant rats causes a specific induction of c-fos in the fetal SCN, raising the possibility of deleterious effects on the developing biological clock. Preliminary evidence presented here suggests that nicotine has a similar effect. In both cases, the response occurs in the fetus but not in adults. This proposal will examine the developmental changes in this response for both nicotine and cocaine, and investigate possible explanations for this unexpected finding.